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The imminent reopening of a crucial border crossing in the Khyber Pass has laid bare one of the vulnerabilities NATO forces are grappling with in prosecuting the war in Afghanistan -- the uneasy, love-hate relationship between Pakistan and the United States.

After nearly two weeks, the Foreign Ministry in Islamabad issued a statement Saturday stating that it will soon reopen the Torkham border post, which lies on a busy supply route to Kabul.

The Pakistani government shuttered the border crossing on Sept. 30, after three of its soldiers were mistakenly killed in an attack by a U.S. helicopter.

The American ambassador to Pakistan, Anne Patterson, apologized for the incident. But the closure has sparked fresh tensions between Washington and Islamabad, partly due to the indispensable role Pakistan plays in supplying the 142,000 coalition troops stationed in Afghanistan, most of whom are American.

The bulk of NATO's fuel and other non-lethal material crosses Pakistan overland from the port of Karachi. Three-quarters of those goods enter Afghanistan via the Khyber Pass, making the Torkham border crossing logistically vital.

"Afghanistan is a very hard place to fight a war because of its physical geographic location," said Sunil Ram, a security expert and professor of land warfare at American Military University. "This is one of the strategic bottlenecks."

Taliban attacks

Aggravating the situation, groups of armed men have attacked tankers laden with NATO fuel on Pakistani soil. The militants are believed to have torched more than 100 tankers in a string of assaults since Oct. 1.

They have targeted fuel trucks that were backed up waiting to cross the Khyber Pass, as well as those making their way to Pakistan's second border crossing to Afghanistan, near the city of Quetta farther south.

The Pakistani Taliban has claimed responsibility for at least two of the assaults. A spokesperson for the group, Azam Tariq, told CNN the fuel trucks were "logistic support for the NATO forces who killed our innocent sisters and brothers in Afghanistan."

However, Ram said private contractors, who are tasked with transporting the fuel, may have spurred the attacks by failing to keep up on payments to the Taliban after the Torkham border post closed.

"The bottom line is, it's about the payoffs," he said, citing sources in military intelligence on both sides of the border. "In the background, the Taliban are saying, ‘Let's get our payoffs back in place and we'll stop blowing your stuff up.'"

The issue of private contractors paying militants has been well documented in Afghanistan. In the latest reported instance, private security forces linked to the Taliban were hired to guard a U.S. base, according to an investigation by the U.S. Senate.

Kamran Bokhari, South Asia director with the global intelligence firm STRATFOR, described the fuel tankers as "a target of opportunity."

"The supply line is just so long, and it runs through several areas where militants are active, that it's not hard for them to hit these trucks," he said. "All you need is a bunch of guys and the ability to torch stuff."

Uneasy allies

The wayward helicopter attack, the subsequent border-crossing closure and fuel tanker attacks have strained already troubled relations between Islamabad and Washington.

Pakistan's high commissioner to Britain, Wajid Shamsul Hasan, said Friday that U.S. authorities were acting on "internal political dynamics" relating to the upcoming midterm elections when they issued a travel alert about militants in Pakistan planning to attack European cities.

On Thursday, the Foreign Ministry in Islamabad criticized the U.S. for what it believes is an increase in the frequency of drone attacks. The Pakistani government has also forbidden cross-border raids by foreign forces, seeing them as violations of the country's sovereignty.

For its part, Washington has accused Islamabad of failing to take action against elements of the Taliban who are keen to fight in Afghanistan but are not hostile to the Pakistani state.

Earlier in the week, a White House report to the Congress warned that Pakistan's military had made a "political choice" to "avoid military engagements that would put it in direct conflict with Afghan Taliban or al Qaeda forces in North Waziristan," according to an unclassified version of the report obtained by Agence France-Presse.

Some officials in Washington suspect the recent fuel tanker attacks were encouraged by elements within Pakistan's intelligence service "to put pressure on the United States not to make incursions into Pakistan," Bokhari said.

He called the current state of U.S.-Pakistan relations "the most tense period between the two sides since this war began."

"But that doesn't mean there will be a breach," he added. "It's kind of like a love-hate relationship."

Pakistan depends on the $2 billion in aid money that flows into its economy from Washington every year. The U.S., in addition to relying on ground supply routes in Pakistan to fuel the NATO war effort, has become increasingly focused on crushing Taliban sanctuaries in Pakistan's tribal areas.

At the heart of the problem, the two governments have failed to agree on "which Taliban groups can be accommodated and which have to be militarily dealt with," Bokhari said.

"That's the clash," he added. "They need to find a middle path, but so far that's not happening."